Method of converting image signal, method of determining original color of color image, and apparatus for converting image signal

ABSTRACT

A new calibration scale is proposed for as a substitute for density values of a densitometer or equivalent neutral densities. A color reproduction range of a color reversal film is set on an xy chromaticity diagram. On the xy chromaticity diagram, there are established three straight lines passing through a chromaticity point corresponding to a standard white illuminant and principal wavelengths relative to primary colors R, G, B. Vertexes of a triangle containing the color reproduction range are determined on the three straight lines. Chromaticity values at the vertexes of said triangle are determined as primary colors R, G, B. Block dye density values c, m, y corresponding to the primary colors R, G, B are determined according to the equations R=10 −c , G=10 −m , B=10 −y . The determined block dye density values c, m, y are used as a new calibration scale. Since the block dye density values have properties similar to conventional density values as compared with colorimetric values, conventional image processing resources can be utilized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of converting adevice-dependent image signal produced by a color scanner (hereinafterreferred to as “scanner”), a digital camera, etc., into adevice-independent image signal, a method of determining an originalcolor in a color reproduction range of a color image that is recorded ona color reversal film, and an apparatus for converting adevice-dependent image signal into a device-independent image signal.

2. Description of the Related Art

For image processing, it has heretofore been customary to covert adevice-dependent image signal produced by a scanner, which is an imageinput device, into a density value on a densitometer or an equivalentneutral density as disclosed in Japanese laid-open patent publicationNo. 6-237373 and Japanese laid-open patent publication No. 6-261208, andprocess an image signal represented by the density value or theequivalent neutral density, i.e., a density-based image signal, forsharpness, set-up, or color correction processing.

In recent years, there has been established a standard color managementtechnique for converting a device-dependent image signal produced by ascanner, a digital camera, etc., which is an image input device, into adevice-independent image signal representing colorimetric values X, Y, Zor L*, a*, b*, processing the device-independent image signal, and thenconverting the processed device-independent image signal into adevice-dependent image signal for use by a printing press, a printer, ora display monitor.

However, the image processing process which processes thedevice-independent image signal based on the colorimetric values failsto utilize the image processing technique that uses density-based imagesignals as conventional image processing resources.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof converting a device-dependent image signal into a noveldevice-independent image signal, which replaces density values on adensitometer or equivalent neutral densities or colorimetric values, amethod of determining an original color of a color image, and anapparatus for converting a device-dependent image signal into adevice-independent image signal.

According to an aspect the present invention, there is provided a methodof converting device-dependent image signals into device-independentimage signals, comprising the step of converting device-dependent imagesignals into device-independent image signals representing densities(density values or density scale) with block dyes. Since thedevice-independent image signals representing densities with block dyeshave properties similar to conventional density Values and equivalentneutral density values, they can be handled with ease, e.g., they haveexcellent organoleptic properties. The device-independent image signalscan highly accurately be converted into colorimetric values throughsimple calculations.

According to another aspect of the present invention, there is alsoprovided a method of converting device-dependent image signals intodevice-independent image signals, comprising the steps of convertingdevice-dependent image signals supplied from an input device which readsan image subject into device-independent image signals representingdensities with block dyes, and converting the device-independent imagesignals into device-dependent image signals for an output device.

The device-dependent image signals may comprise R, G, B signals or C, M,Y signals, and the device-independent image signals may comprise C, M, Ysignals representing densities with block dyes. Therefore, R, G, Bsignals and C, M, Y signals outputted from an image input device such asa digital camera, a scanner, or the like may be handled as C, M, Ysignals representing densities with block dyes.

According to still another aspect of the present invention, there isfurther provided a method of determining primary colors of a colorimage, comprising the steps of setting a color reproduction range of thecolor image on an xy chromaticity diagram, setting three straight linesextending through a chromaticity point corresponding to a standard whiteilluminant on the xy chromaticity diagram and principal wavelengthsrelative to primary colors in the color reproduction range, determiningthe vertexes of a triangle containing the color reproduction range onthe three straight lines, and determining choromaticity values at thevertexes of the triangle as primary colors. Density values of block dyescan be calculated from the determined primary colors.

The color image may be carried on a color reversal film or a reflectivecolor print.

According to yet another aspect of the present invention, there isfurther provided an apparatus for converting device-dependent imagesignals into device-independent image signals, comprising an inputconverter for converting device-dependent image signals intodevice-independent image signals representing densities with block dyes.

With the above apparatus, because the device-independent image signalsrepresenting densities with block dyes have properties similar toconventional density values and equivalent neutral density values, theycan be handled with ease, e.g., they have excellent organolepticproperties. The device-independent image signals can highly accuratelybe converted into colorimetric values through simple calculations.

According to yet still another aspect of the present invention, there isprovided an apparatus for converting device-dependent image signals intodevice-independent image signals, comprising an input converter forconverting device-dependent image signals supplied from an input devicewhich reads an image subject into device-independent image signalsrepresenting densities with block dyes, and an output converter forconverting the device-independent image signals into device-dependentimage signals for an output device.

In the above apparatus, the device-dependent image signals may compriseR, G, B signals or C, M, Y signals, and the device-independent imagesignals may comprise C, M, Y signals representing densities with blockdyes. Therefore, R, G, B signals and C, M, Y signals outputted from animage input device such as a digital camera, a scanner, or the like maybe handled as C, M, Y signals representing densities with block dyes.

According to a further aspect of the present invention, there isprovided an apparatus for converting device-dependent image signals intodevice-independent image signals, comprising a plurality ofone-dimensional conversion tables for processing device-dependent imagesignals supplied from an input device which reads an image subject, withrespective predetermined functions, a table selector for selecting oneof the one-dimensional conversion tables which is optimum for the inputdevice, and an input converter for converting the device-dependent imagesignals processed by the one-dimensional conversion table which isselected by the table selector, into device-independent image signalsrepresenting densities with block dyes. One of the one-dimensionalconversion tables can be selected depending on the input device, whichmay comprise a digital camera, a scanner, or the like. Thus, theobtained device-independent image signals representing densities withblock dyes well match the device-dependent image signals supplied fromthe input device.

The table selector may comprise means for using block dye densitiesdetermined from a reference color chart as target values, means forprocessing, as input values, image signals which are produced by readingthe reference color chart with the input device and processed by theone-dimensional conversion tables, according to a predeterminedpolynomial, thereby to produce calculated values, and means forselecting one of the one-dimensional conversion tables which outputs theinput values corresponding to those of the calculated values which areclosest to the target values, as the one-dimensional conversion tablewhich is optimum for the input device.

The predetermined polynomial may comprise a polynomial based on aregression analysis.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which preferredembodiments of the present invention are shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrative of block dyes;

FIG. 2 is a block diagram of an image processing system according to thepresent invention;

FIG. 3 is a detailed block diagram of a color processor in the imageprocessing system shown in FIG. 2;

FIG. 4 is a flowchart of an operation sequence of the image processingsystem shown in FIG. 2;

FIG. 5 is a diagram illustrative of details of an IT8 chart;

FIG. 6 is a flowchart of an operation sequence of a process ofdetermining a block dye density;

FIG. 7 is a chromaticity diagram illustrative of a color reproductionrange of a color reversal film;

FIG. 8 is a chromaticity diagram illustrative of a process of threestraight lines;

FIG. 9 is a chromaticity diagram illustrative of a process ofdetermining the vertexes of a triangle;

FIG. 10 is a block diagram of an apparatus for converting an imagesignal;

FIG. 11 is a block diagram of another apparatus for converting an imagesignal; and

FIG. 12 is a block diagram of still another apparatus for converting animage signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior to describing embodiments of the present invention, block dyes andthe relationship of conversion between the block dyes and colorimetricvalues will be described below for an easier understanding of theembodiments of the present invention.

Block dyes are described in “Fundamentals of color reproductionengineering”, written by Noboru Ohta, published by Corona Co., Ltd.,Sep. 10, 1997, 1st edition, 1st printing, pages 105-107 (hereinafterreferred to as “literature 1”).

Block dyes are dyes whose spectral absorption curves are of block shape.As shown in FIG. 1, a block dye Y (yellow) has a density (also referredto as “density value” or “density scale”) D having a uniform densityvalue y (D=y) between wavelengths λ1 and λ2. A block dye M (magenta) hasa density D having a uniform density value m (D=m) between wavelengthsλ2 and λ3. A block dye C (cyan) has a density D having a uniform densityvalue c (D=c) between wavelengths λ3 and λ4. As well known in the art,the density D can be expressed by the following equation (1):D=−log ₁₀ T←→T=10^(−D)  (1)where T (λ) represents spectral transmittance.

According to the present invention, the density values c, m, y of theblock dyes C, M, Y which express the colors of a color image (subject)recorded on a color reversal film, a reflective color print, etc. aredensity values converted (calibrated) into a novel device-dependentimage signal which replaces density values on a densitometer orequivalent neutral densities or colorimetric values.

The wavelength ranges of the block dyes C, M, Y do not need to share awavelength between the block dyes Y, M or between the block dyes M, C,as shown in FIG. 1. Rather, the wavelength ranges of the block dyes Y, Mor the block dyes M, C may overlap each other, or may be spaced fromeach other by a wavelength gap therebetween.

A colorimetric value X is expressed by the equation (2), and can bedeveloped according to the equations (3), (4) in view of the aboveproperties of the block dyes C, M, Y. $\begin{matrix}\begin{matrix}{{X = {\int{{T(\lambda)} \cdot {P(\lambda)}}}}{{x(\lambda)}{\mathbb{d}\lambda}}} \\{= {{10^{- c}{\int{{{P(\lambda)} \cdot {x(\lambda)}}{\mathbb{d}\lambda}}}} + 10^{- m}}} \\{{\int{{{P(\lambda)} \cdot {x(\lambda)}}{\mathbb{d}\lambda}}} + {10^{- Y}{\int{{{P(\lambda)} \cdot {x(\lambda)}}{\mathbb{d}\lambda}}}}} \\{= {{10^{- c} \cdot {Xr}} + {10^{- m} \cdot {Xg}} + {10^{- y} \cdot {Xb}}}}\end{matrix} & (2) \\\quad & (3) \\\quad & (4)\end{matrix}$

In the equation (2), the range of the integral ∫ represents the visiblewavelength range. In the equation (3), the range of the integral ∫ ofthe first term on the right side represents the range between λ3 and λ4,the range of the integral ∫ of the second term of the right siderepresents the range between λ2 and λ3, and the range of the integral ∫of the third term of the right side represents the range between λ1 andλ2.

In the equation (4), Xr, Xg, Xb represent Xr=∫P(λ)·x(λ)dλ (the range ofthe integral ∫ represents the range between λ3 and λ4), Xg=∫P(λ)·x(λ)dλ(the range of the integral ∫ represents the range between λ2 and λ3),Xb=∫P(λ)·x(λ)dλ (the range of the integral ∫ represents the rangebetween λ1 and λ2), respectively.

Therefore, the block dyes C, M, Y can easily be converted intocolorimetric values X, Y, Z by matrix and product sum operations, ratherthan integral operations, according to the following matrix equation(5): $\begin{matrix}{\begin{bmatrix}X \\Y \\Z\end{bmatrix} = {\begin{bmatrix}X_{r} & X_{g} & X_{b} \\Y_{r} & Y_{g} & Y_{b} \\Z_{r} & Z_{g} & Z_{b}\end{bmatrix}\begin{bmatrix}10^{- c} \\10^{- m} \\10^{- y}\end{bmatrix}}} & (5)\end{matrix}$

Since the colorimetric values X, Y, Z and colorimetric values L*, a*, b*can uniquely be converted between each other according to colorimetricequations, the term “colorimetric values” used herein represent eitherthe X, Y, Z values or the L*, a*, b* values.

The elements of the matrixes according to the equation (5) can bedetermined irrespective of wavelengths, as described below.

It is assumed that primary colors (which may actually include imaginaryprimary colors that do not exit) R, G, B are equal to the transmittances10 ^(−c), 10 ^(−m), 10 ^(−y), respectively, of the block dyes C, M, Y inthe equation (5), as indicated by the following equation (6):R=10^(−c), G=10^(−m), B=10^(−y)  (6)

By substituting the equation (6) in the equation (5), the followingequation (7) is obtained: $\begin{matrix}{\begin{bmatrix}X \\Y \\Z\end{bmatrix} = {\begin{bmatrix}X_{r} & X_{g} & X_{b} \\Y_{r} & Y_{g} & Y_{b} \\Z_{r} & Z_{g} & Z_{b}\end{bmatrix}\begin{bmatrix}R \\G \\B\end{bmatrix}}} & (7)\end{matrix}$

According to the equation (7), the subtractive mixture of the block dyesC, M, Y can be replaced with the additive mixture, and hence can behandled in the same manner as the additive mixture.

Therefore, if imaginary primary color R, G, B values with respect to acolor image recorded on a color reversal film or the like are initiallydetermined, then density values c, m, y of imaginary block dyes C, M, Ycorresponding to the primary color R, G, B values can be estimated,i.e., calculated, from the primary color R, G, B according to theequation (6).

In the embodiments described below, imaginary primary color R, G, Bvalues are initially determined, and then density values c, m, y ofimaginary block dyes C, M, Y corresponding to the primary color R, G, Bvalues are determined. A process of determining the elements Xr, Xg, Xb,Yr, Yg, Yb, Zr, Zg, Zb of the matrix according to the equation (7) whenthe primary color R, G, B values are initially determined is disclosedin “Fundamentals of calorimetric engi-′neering”, written by Gakuo Ikeda,published by Corona Co., Ltd., Oct. 15, 1989, 6th printing, pages125-130 (hereinafter referred to as “literature 2w).

The process of determining the elements Xr, Xg, Xb, Yr, Yg, Yb, Zr, Zg,Zb will be described below.

A general formula for converting primary color R, G, B values intocalorimetric values X, Y, Z is given by the following equation (8):$\begin{matrix}{\begin{bmatrix}R \\G \\B\end{bmatrix} = {\begin{bmatrix}R_{X} & R_{Y} & R_{Z} \\G_{X} & G_{Y} & G_{Z} \\B_{X} & B_{Y} & B_{Z}\end{bmatrix}\begin{bmatrix}X \\Y \\Z\end{bmatrix}}} & (8)\end{matrix}$

In the equation (8), the values of the matrix elements Rx, Ry, Rz, Gx,Gy, Gz, Bx, By, Bz, i.e., the coefficients of the elements X, Y, Z ofthe right matrix on the right side, can be determined by specifying x, Ychromaticity values of the primary colors R, G, B on the left side andx, Y chromaticity values of a standard illuminant (white) according tothe CIE, as described in the above pages of the literature 2.

The equation (8) can be modified into the following equation (9):$\begin{matrix}{\begin{bmatrix}X \\Y \\Z\end{bmatrix} = {\begin{bmatrix}R_{X} & R_{Y} & R_{Z} \\G_{X} & G_{Y} & G_{Z} \\B_{X} & B_{Y} & B_{Z}\end{bmatrix}^{- 1}\begin{bmatrix}R \\G \\B\end{bmatrix}}} & (9)\end{matrix}$

If the left inverse matrix on the right side of the equation (9) and theleft matrix on the right side of the equation (7) are made equal to eachother as shown by the equation (10) below, then the elements Xr, Xg, Xb,Yr, Yg, Yb, Zr, Zg, Zb of the matrixes according to the equation (7)when the primary color R, G, B values are initially determined can bedetermined. $\begin{matrix}{\begin{bmatrix}X_{r} & X_{g} & X_{b} \\Y_{r} & Y_{g} & Y_{b} \\Z_{r} & Z_{g} & Z_{b}\end{bmatrix} = \begin{bmatrix}R_{X} & R_{Y} & R_{Z} \\G_{X} & G_{Y} & G_{Z} \\B_{X} & B_{Y} & B_{Z}\end{bmatrix}^{- 1}} & (10)\end{matrix}$

The block dyes and the relationship of conversion between the block dyesand colorimetric values have been described above.

Now, an image processing system 10 according to the present inventionwill be described below.

FIG. 2 shows in block form the image processing system 10.

As shown in FIG. 2, the image processing system 10 basically comprises ascanner 14 as an image input device (also referred to as” image readingdevices or “image input device”), and an image processor 15 connected tothe scanner 14.

The scanner 14 scans a color reversal film 12 as a subject with anoptical system, and outputs device-dependent image signals (R, G, B)representing color-separated pixel signals to the image processor 15.

The image processor 15 converts the device-dependent image signals (R,G, B) from the scanner 14 into device-dependent image signals (C, M, Y,K), for example, for printing.

The image processor 15 comprises a computer for carrying out a series ofcalculations or a data processing sequence according to a program.Specifically, the image processor 15 comprises a computer unit having aCPU, a memory, etc., an input device including a mouse, a keyboard, etc.connected to the computer unit, and a display unit such as a CRT displayor the like connected to the computer unit.

The image processor 15 has a linear conversion table (also referred toas “one-dimensional LUT”) 16 as a one-dimensional LUT (Look-Up Table)for converting the device-dependent image signals (R, G, B) from thescanner 14 into image signals (C, M, Y)=(Ca, Ma, Ya) as intermediatevalues, a logarithmic conversion table (also referred to as“one-dimensional LUT”) 18 as a one-dimensional LUT for converting thedevice-dependent image signals (R, G, B) from the scanner 14 into imagesignals (C, M, Y)=(Cb, Mb, Yb) as intermediate values, and a luminancelogarithmic conversion table (also referred to as “one-dimensional LUT”)20 as a one-dimensional LUT for converting the device-dependent imagesignals (R, G, B) from the scanner 14 into image signals (C, M, Y)=(Cc,Mc, Yc) as intermediate values.

These one-dimensional LUTs 16, 18, 20 are tables inserted for eitherconverting signals which cannot easily be approximated by a polynomial,described later on, according to predetermined functions thereby toincrease the accuracy of approximation by the polynomial, or reducingthe number of terms of the polynomial if the accuracy of approximationmay remain unchanged.

The linear conversion table 16 has a function to linearly convert thesupplied image signals (R, G, B) according to respective linearequations.

The logarithmic conversion table 18 has a function to logarithmicallyconvert the image signals obtained as luminance signals into imagesignals as density signals.

The luminance logarithmic conversion table 20 has a function to convertan L* value in the CIELAB color space into a luminance and thereafterlogarithmically convert the luminance.

The image signals (Ca, Ma, Ya), (Cb, Mb, Yb), (Cc, Mc, Yc) asintermediate values outputted respectively from the one-dimensional LUTs16, 18, 20 are supplied to a table selecting calculator 22 for selectingeither one of the one-dimensional LUTs 16, 18, 20.

The table selecting calculator 22 performs given calculations usingblock dye density values (C′, M′, Y′) generated by a target generator 24as target values, selects one of the one-dimensional LUTs 16, 18, 20which outputs image signals closest to the target values when'the tableselecting calculator 22 calculates the polynomial using the imagesignals (Ca, Ma, Ya), (Cb, Mb, Yb), (Cc, Mc, Yc) as intermediate values,switches a table selector 28 which comprises a multiplexer, and holdscoefficients Bm of the determined polynomial in a coefficient holder 26as a memory.

The image processor 15 also has a DDC (Device-Dependent Color)/DIC(Device-Independent Color) converter 30 as a polynomial calculator forconverting device-dependent image data into device-independent imagedata. The DDC/DIC converter 30 converts the device-dependent imagesignals from one of the one-dimensional LUTs 16, 18, 20 which has beenselected by the table selector 28, into block dye density values (C, M,Y) as device-independent image signals according to a given polynomialwhose cofficients Bm are supplied from the coefficient holder 26.

The block dye density values (C, M, Y) are then converted by a colorprocessor 32 into device-dependent-image signals (C, M, Y, K) ashalftone dot % (halftone dot area ratio) signals if the output device(image output device) comprises a printing press, for example.

In the image processor 15, as described above, the device-dependentimage signals (R, G, B) outputted from the scanner 14 via one of theone-dimensional LUTs 16, 18, 20 and the table selector 28 to the DDC/DICconverter 30, and then converted into device-dependent image signals (C,M, Y, K) for printing by the DDC/DIC converter 30 and the colorprocessor 32.

Actually, when the device-dependent image signals (R, G, B) outputtedfrom the scanner 14 or the like are supplied to the image processor 15,the table selecting calculator 22 selects the signals from one of theone-dimensional LUTs 16, 18, 20 which approximate the block dye densityvalues (C′, M′, Y′) as target values most accurately. Thereafter, acombined LUT generator 34 combines or merges the selected one of theone-dimensional LUTs 16, 18, 20, the DDC/DIC converter 30, and the colorprocessor 32 into a combined LUT, and stores the combined LUT into acombined LUT setting unit 36 as a memory. The device-dependent imagesignals (R, G, B) outputted from the scanner 14 is converted into thedevice-dependent image signals (R, G, B) for printing, by thedevice-dependent image signals (R, G, B) stored in the combined LUTsetting unit 36, i.e., one LUT.

The coefficient holder 26 is required to comprise an electricallyerasable programmable ROM such as a flash memory or the like. Thecombined LUT setting unit 36 may comprise a RAM such as a volatilememory incorporated as a working memory in the computer.

The components of the image processor 15 which range from theone-dimensional LUTs 16, 18, 20 to the DDC/DIC converter 30 perform aninput converting process, the color processor 32 performs an outputconverting process.

The color processor 32 which performs an output converting process willbe described as to its arrangement and operation in detail below. Thecolor processor 32 generates a density signal K based on the setting ofdensities including highlights (HL) and shadows (SD), the gradationconversion for each color, the adjustment of the gray balance, colorcorrection, and UCR (Under Color Removal) according to image processingconditions that are specified by the operator using input deviceincluding a mouse, a keyboard, etc. The color processor 32 may bearranged as shown in FIG. 3, for example.

As shown in FIG. 3, the color processor 32 comprises an HL/SD densitysetting unit 40, a tone curve setting unit 42, a signal rearrangementoperating unit 44, a UCR operating unit 46, a K plate generator 48, ahalftone dot % gray balance generator 50, a halftone dot % setting unit52, and a color corrector 54. The color processor 32 generates andoutputs color-processed device-dependent image signals (C, M, Y, K)based on instruction data a1-a7 supplied from the operator via the inputdevice to the various components 40, 42, 46, 48, 50, 54, 52, and theblock dye density values (block dye density value signals) (C, M, Y) asdevice-independent image signals outputted from the DDC/DIC converter 30(see FIG. 2) and supplied to the HL/SD density setting unit 40.

The HL/SD density setting unit 40 normalizes the densities at thehighlight and shadow setting points of the supplied block dye densityvalues (C, M, Y) with the density setting values of the instruction dataa1 relative to the output device.

The tone curve setting unit 42 selects a basic tone curve as one ofimage conversion functions based on the instruction data a2, or sets atone curve by correcting the basic tone curve with a curve correctioncoefficient indicated by the instruction data a2. The tone curve settingunit 42 converts the gradations of the image signals (C, M, Y)normalized by the HL/SD density setting unit 40 with respect to theimage signal C relative to cyan according to the set tone curve.

The signal rearrangement operating unit 44 compares the magnitudes ofthe image signals (C, M, Y) from the HL/SD density setting unit 40 todetermine a maximum value max and a minimum value min.

The UCR operating unit 46 calculates a UCR quantity with respect to theimage signals (C, M, Y) according to the maximum value max and theminimum value min from the signal rearrangement operating unit 44 andgray width control data and UCR intensity data based on the instructiondata a3. The UCR quantity is subtracted from the image signals (C, M, Y)prior to the UCR process by a subtractor 43. If the UCR process isreplaced with a UCR (Under Color Addition) process, then the UCRintensity data may be set to a negative value, and the obtained UCRquantity may be added to the image signals (C, M, Y) prior to the UCAprocess.

The K plate generator 48 calculates an image signal K according to themaximum value max and the minimum value min from the signalrearrangement operating unit 44 and K plate gray width control data andK plate generating curve correcting coefficient data based on theinstruction data a4.

The halftone dot % gray balance generator 50 converts the image signals(C, M, Y) into halftone dot % signals (C, M, Y), which turn the imagesignals (C, M, Y) into gray signals, and also converts the image signalK into a halftone dot % signal K, according to gray balance data basedon the instruction data a5.

The color corrector 54 determines hue signals, lightness signals, andchroma signals from the image signals (C, M, Y) supplied from the HL/SDdensity setting unit 40, and determines corrective quantities AC, AM,AY, AK for the image signals (C, M, Y, K) as halftone dot % according tocorrection coefficients based on the instruction data a6. The correctivequantities ΔC, ΔM, ΔY, ΔK are then added respectively to the halftonedot % signals (C, M, Y, K) outputted from the halftone dot % graybalance generator 50 by an adder 51.

The halftone dot % setting unit 52 further corrects the halftone dot %signals (C, M, Y, K) to which the corrective quantities ΔC, ΔM, ΔY, ΔKhave been added, according to highlight and shadow halftone dot %setting values based on the instruction data a7.

The color processor 32 converts the device-independent image signals (C,M, Y) supplied to the HL/SD density setting unit 40 into thedevice-dependent image signals (C, M, Y, K) outputted from the halftonedot % setting unit 52.

Therefore, the color processor 32 which operates in the manner describedabove converts the block dye density values (C, M, Y) supplied asdevice-independent image signals from the are DDC/DIC converter 30 intothe device-dependent halftone dot % signals (C, M, Y, K). The colorprocessor 32 may be constructed as a single look-up table representingthe relationship of conversion between the block dye density values (C,M, Y) and the halftone dot % signals (C, M, Y, K). The look-up tablethus constructed is indicated as a color processing LUT 56 by thetwo-dot-and-dash line in FIG. 3.

A process of determining the coefficients Bm of the polynomial forconverting the device-dependent image signals outputted from the inputdevice such as the scanner 14 or the like into the device-independentimage signals with high accuracy, a process of selecting an optimum oneof the one-dimensional LUTs 16, 18, 20 used to determine thecoefficients Bm of the polynomial, and the input converting process ofconverting the device-dependent image signals into thedevice-independent image signals will be described below with referenceto FIG. 4.

In step S1 shown in FIG. 4, the scanner 14 scans and reads anANSI/IT8.7/1-1993 chart (hereinafter referred to as “IT8 chart”) CT forcolor reversal films, as an input color target according to the ANSI(American National Standard Institute) to obtain device-dependent imagesignals (R, G, B) for each of patches of the IT8 chart CT.

The IT8 chart CT, which is described in pages 56-59 of the literature 1,will be summarized as follows:

The IT8 chart CT as an input target is used to convert color signalsfrom a color reversal film into device-independent signals (L*, a*, b*),and is arranged as shown in FIG. 5.

As seen from FIG. 5, the IT8 chart CT has 144 color solid colors inlines A-L and columns 1-12, primary color scales of C, M, Y, K, R, G, Bin lines A-L and columns 13-19, manufacturer's inherent colors in linesA-L and columns 20-22, and a lower gray scale of 22 steps. The colorsolid colors in the columns 1-12 are colors selected substantiallyuniformly from a common color solid which is common to various realcolor reversal films.

The primary color scales in the columns 13-19 are determined as follows:Gray densities ranging from minimum to maximum densities are equallydivided, producing a K scale. The K scale is divided to determineprimary color scales of C, M, Y. These primary colors are superposed incombinations of two to determine primary colors of R (=M+Y), G (=Y+C), B(=C+M).

In the present embodiment, image signals (R, G, B) (hereinafter alsoreferred to as “R, G, B values”) which are device-dependent imagesignals representing the color patches of a total of 288 colors of theIT8 chart CT are determined by scanning and reading the IT8 chart CTwith the scanner 14.

In step S2, the image signals (R, G, B) produced by the scanner 14 aresupplied to the one-dimensional LUTs 16, 18, 20, which generate imagesignals (Ca, Ma, Ya), image signals (Cb, Mb, Yb), and image signals (Cc,Mc, Yc), respectively, that are supplied to the table selectingcalculator 22.

In step S3, the target generator 24 determines block dye density values(C′, M′, Y′) from colorimetric value signals (X, Y, Z) of the patchesattached to the IT8 chart CT, and supplies the determined block dyedensity values (C′, M′, Y′) as target values to the table selectingcalculator 22.

A process of determining the block dye density values (C′, M′, Y′) fromcolorimetric value signals (X, Y, Z) of the patches attached to the IT8chart CT in step S3 will be described in detail below with reference toFIG. 6.

For determining the block dye density values (C′, M′, Y′) fromcolorimetric value signals (X, Y, Z), it is necessary to know thevaluses of the elements Xr, Xg, Xb, Yr, Yg, Yb, Zr, Zg, Zb of the leftmatrix on the right side of the equation (7) (the equation (5)).Specifically, the equations (5) and (7) can be modified respectivelyinto the following equations (12) and (11). $\begin{matrix}{\begin{bmatrix}R \\G \\B\end{bmatrix} = {\begin{bmatrix}X_{r} & X_{g} & X_{b} \\Y_{r} & Y_{g} & Y_{b} \\Z_{r} & Z_{g} & Z_{b}\end{bmatrix}^{- 1}\begin{bmatrix}X \\Y \\Z\end{bmatrix}}} & (11) \\{\begin{bmatrix}10^{- c} \\10^{- m} \\10^{- y}\end{bmatrix} = {\begin{bmatrix}X_{r} & X_{g} & X_{b} \\Y_{r} & Y_{g} & Y_{b} \\Z_{r} & Z_{g} & Z_{b}\end{bmatrix}^{- 1}\begin{bmatrix}X \\Y \\Z\end{bmatrix}}} & (12)\end{matrix}$

When the calorimetric value signals (X, Y, Z) are substituted in theequation (12), block dye density values (c, m, y) on the left side ofthe equation (12) can be determined from the equation (12).

As described above, in order to determine the values of the elements Xr,Xg, Xb, Yr, Yg, Yb, Zr, Zg, Zb of the matrix, stated otherwise, in orderto determine block dye density values (block dye density scale), it isnecessary to determine imaginary primary colors R, G, B.

In a process of setting a color image color reproduction range in stepS11 shown in FIG. 6, a color reproduction range 100 of the colorreversal film 12 is set or plotted which is enclosed by a dotted line onthe xy chromaticity diagram according to the CIE, as shown in FIG. 7.Plotted points 102 in the color reproduction range 100 are points wherea color chart having a maximum number of (e.g., 3375) color patches thatcan be expressed as colors on a color reversal film was measured by theapplicant of the present invention with a colorimeter. It is preferableto measure the color chart at many points in the vicinity of principalwavelengths, stated otherwise, those wavelengths including the colors R,G, B and their complementary colors C, M, Y, for the purpose of settingblock dye density values with greater accuracy. As well known in theart, an x chromaticity value is calculated from the colorimetric valuesX, Y, Z measured by the colorimeter according to x=X/(X+Y+Z), a ychromaticity value is calculated from the calorimetric values X, Y, Zaccording to y=Y/(X+Y+Z), and z chromaticity value is calculated fromthe calorimetric values X, Y, Z according to Z=1−x−y=Z/(X+Y+Z).

In FIG. 7, the range of an envelope formed by plotted points 104measured by the applicant represents an actually existing color range106.

In a process of setting straight lines in step S12, as shown in FIG. 8,a chromaticity point 118 corresponding to a standard white illuminant isset or plotted on the xy chromaticity diagram, and three straight lines111, 112, 113 are established which extend outwardly from thechromaticity point 118 and pass through the principal wavelengthsrelative to the primary colors in the color reproduction range 100 setin step S11. In the present embodiment, a chromaticity value (x,y)=(0.3457, 0.3586) of a supplementary standard illuminant D₅₀ accordingto the CIE is used as the standard white illuminant. Thus, in FIG. 8,the coordinates of the chromaticity point 118 are represented by (x,y)=(0.3457, 0.3586).

In a process of determining the vertexes of a triangle containing thecolor reproduction range in step S13, as shown in FIG. 9, vertexes 121,122, 123 of a triangle 116 containing the color reproduction range 100of the color reversal film are determined on the three straight lines111, 112, 113. If the triangle 116 containing the color reproductionrange 100 is of a size such that the area of the triangle 116 is assmall as possible or minimum, then the accuracy of approximation isincreased.

In a process of determining primary colors in step S14, xy chromaticityvalues of the vertexes 121, 122, 123 of the triangle 116 are read fromthe chromaticity diagram shown in FIG. 9), and determined aschromaticity values of imaginary primary colors R, G, B. The imaginaryprimary colors R, G, B are involved because the chromaticity values ofthe imaginary primary colors R, G are present outside the actual colorrange 106.

By substituting the values of the primary colors R, G, B for R=10^(−c),B=10^(−m), G=10^(−y) in the equation (6), density Values c, m, y of theblock dyes in the equation (6) are calculated.

If the values of the primary colors R, G, B are determined as xychromaticity values, then density values c, m, y of the block dyes canbe calculated according to the equations of z=1−x−y, x=X/(X+Y+Z),y=Y/(X+Y+Z) and the equation (12).

Based on the chromaticity values of the primary colors R, G, Bdetermined in step S14 and the chromaticity value of D₅₀, values of theelements R_(x), R_(y), R_(z), G_(x), G_(y), G_(z), B_(x), B_(y), B_(z)of the left matrix on the right side of the equation (8) are determined.As described above with respect to the equations (9) and (10), theelements Xr, Xg, Xb, Yr, Yg, Yb, Zr, Zg, Zb of the left matrix on theright side of the equation (7) can be determined, and hence values ofthe left inverse matrix on the right side of the equation (12) can bedetermined.

With the equation (12) set in the target generator 24, it is possible todetermine block dye density values (C′, M′, Y′) from the colorimetricvalues (X, Y, Z) of the patches attached to the IT8 chart CT, and supplythe determined block dye density values (C′, M′, Y′) as target values tothe table selecting calculator 22, in step S3.

In step S4, the table selecting calculator 22 performs a regressionanalysis. In the regression analysis, the relationship between the inputvalues (Ca, Ma, Ya), (Cb, Mb, Yb), (Cc, Mc, Yc) as device-dependentimage signals and the above target values (C′, M′, Y′), i.e., the blockdye density values as device-dependent image signals, is expressed by apolynomial according to the following equation (13): $\begin{matrix}{{C^{\prime}{a\left( {{Ca},{Ma},{Ya}} \right)}} = {\sum\limits_{i = 0}^{n}{\sum\limits_{j = 0}^{i}{\sum\limits_{k = 0}^{i - j}{A_{{jk}{({i - j - k})}}{Ca}^{j}{Ma}^{k}{Ya}^{({i - j - k})}}}}}} & (13)\end{matrix}$

In the equation (13), the target value C′ corresponding to the inputvalues (Ca, Ma, Ya) is expressed as C′a (Ca, Ma, Ya). Similarly, in thedescription which follows, target values M′, Y′ correspondingrespectively to the input values (Cb, Mb, Yb), (Cc, Mc, Yc) areexpressed as M′a (Cb, Mb, Yb), Y′a (Cc, Mc, Yc), respectively.

In the equation (13), A_(jk(i-j-k)) represents a coefficient, and nrepresents the degree of the polynomial and is determined depending onthe required accuracy. In the present embodiment, the degree n is n=7.

The equation (13) can be developed in the following equation (14):$\begin{matrix}\begin{matrix}{{C^{\prime}{a\left( {{Ca},{Ma},{Ya}} \right)}} = {A_{000} + {A_{001}{Ya}} + {A_{010}{Ma}} + {A_{100}{Ca}} +}} \\{{A_{002}{Ya}^{2}} + {A_{011}{MaYa}} + {A_{020}{Ma}^{2}} +} \\{{A_{101}{CaYa}} + {A_{110}{CaMa}} + {A_{200}{ca}^{2}} + \ldots}\end{matrix} & (14)\end{matrix}$

The target values M′, Y′ can also be expressed according to similarequations.

For reasons of convenience, the equation (14) is replaced with thefollowing equation (15): $\begin{matrix}\begin{matrix}{{C^{\prime}{a\left( {{Ca},{Ma},{Ya}} \right)}} = {B_{0} + {B_{1}{Ya}} + {B_{2}{Ma}} + {B_{3}{Ca}} + {B_{4}{Ya}^{2}} +}} \\{{B_{5}{MaYa}} + {B_{6}{Ma}^{2}} + {B_{7}{CaYa}} +} \\{{B_{8}{CaMa}} + {B_{9}{Ca}^{2}} + \ldots}\end{matrix} & (15)\end{matrix}$

In order to effect a regression analysis on the equation (15), theequation (15) is replaced with the following equation (16) where thenumber of terms of the polynomial is set to n′{n′=(n+1)(n+2)(n+3)/6=(8×9×10)/6=120}, C′a is regarded as an objectvariable, Xm (m=0, 1, . . . , n′−1) as explanatory variables, and Bm(m=0, 1, . . . , n′−1) as coefficients: $\begin{matrix}\begin{matrix}{{C^{\prime}{a\left( {{Ca},{Ma},{Ya}} \right)}} = {{B_{0}X_{0}} + {B_{1}X_{1}} + \cdots\quad + {BmXm} + \cdots\quad +}} \\{{Bn}^{\prime} - {1\quad{Xn}^{\prime}} - 1}\end{matrix} & (16)\end{matrix}$

A matrix equation for effecting a regression analysis on the objectvariable C′a with respect to all the 288 patches of the IT8 chart CT isexpressed by the equation (17) shown below. In the equation (17), aregression analysis is effected on object variables C′a₀, C′a₁, . . . ,C′a_(m), . . . , C′a₂₈₇ corresponding to all the patches of the IT8chart CT, thereby determining the coefficients Bm (m=0, 1, . . . ,N′−1). $\begin{matrix}{\begin{bmatrix}{C^{\prime}a_{0}} \\{C^{\prime}a_{1}} \\\vdots \\\vdots \\\vdots \\{C^{\prime}a_{287}}\end{bmatrix} = {\begin{bmatrix}X_{00} & X_{10} & \cdots & {{Xn}^{\prime} - 1_{0}} \\X_{00} & X_{11} & \cdots & {{Xn}^{\prime} - 1_{1}} \\\vdots & \vdots & \vdots & \vdots \\\vdots & \vdots & \vdots & \vdots \\\vdots & \vdots & \vdots & \vdots \\X_{0287} & X_{1287} & \cdots & {{Xn}^{\prime} - 1_{287}}\end{bmatrix}\begin{bmatrix}B_{0} \\B_{1} \\\vdots \\\vdots \\{{Bn}^{\prime} - 1}\end{bmatrix}}} & (17)\end{matrix}$

If the coefficients Bm are determined, then the regression analysis instep S4 is finished.

After the coefficients Bm are determined, the table selecting calculator22 determines a conversion formula, i.e., a polynomial, for convertingthe input values (Ca, Ma, Ya) into target values (C′a, M′a, Y′a)according to the equations (15), (16). The polynomial uses the linearconversion table 16 as a one-dimensional LUT.

Similarly, the table selecting calculator 22 determines a conversionformula (polynomial) for converting the input values (Cb, Mb, Yb) intotarget values (C′b, M′b, Y′b) and a conversion formula (polynomial) forconverting the input values (Cc, Mc, Yc) into target values (C′c, M′c,Y′c) using the remaining conversion tables, i.e., the logarithmicconversion table 18 and the luminance logarithmic conversion table 20,each as one-dimensional LUT.

In step S6, target values (C′a, M′a, Y′a), (C′b, M′b, Y′b), (C′c, M′c,Y′c) are calculated with respect to the input values (Ca, Ma, Ya), (Cb,Mb, Yb), (Cc, Mc, Yc) according to the equation (14) whose coefficientvalues are determined.

In step S7, the calculated target values (C′a, M′a, Y′a), (C′b, M′b,Y′b), (C′c, M′c, Y′c) are compared with the block dye density values(C′, M′, Y′) determined as target values in step S3.

In step S8, based on the compared result, those values of the calculatedtarget values (C′a, M′a, Y′a), (C′b, M′b, Y′b), (C′c, M′c, Y′c) whichare closest on the average to the target values (C′, M′, Y′) areselected, and one of the one-dimensional LUTs 16, 18, 20 correspondingto the selected values is determined.

In step S9, the table selecting calculator 22 switches the tableselector 28 to read output image signals from one of the one-dimensionalLUTs 16, 18, 20 which is determined depending on the scanner 14 as theinput device, holds the coefficients Bm corresponding to the scanner 14in the coefficient holder 26, and sets the polynomial according to theequation (15) that corresponds to the scanner 14 in the DDC/DICconverter 30.

When the scanner 14 is actually selected as the input device, the imageprocessor 15 switches the table selector 28 based on the result ofapproximating calculations to select one of the one-dimensional LUTs 16,18, 20 corresponding to the scanner 14, in response to an instructionsupplied from the operator via the input device or automaticallyaccording to communications with the scanner 14, and sets thecoefficients Bm, which correspond to the scanner 14, from thecoefficient holder 26 in the polynomial according to the equation (15)that is set in the DDC/DIC converter 30. The image processor 15 thencombines or merges the selected one-dimensional LUT, the polynomialwhose coefficients Bm are set, and the color processing LUT 56 as thecolor processor 32 with the combined LUT generator 34, and sets themerged LUT into the combined LUT setting unit 36.

With the merged LUT thus set, it is possible to easily convertdevice-dependent image signals (R, G, B) outputted from the scanner 14as the input device when a color reversal film 12 as a subject is readby the scanner 14 into device-dependent image signals (C, M, Y, K)suitable for use by a printing press as the output device.

In the present embodiment, as described above, the image processor 15performs the input converting process of converting device-dependentimage signals (R, G, B) supplied from the scanner 14 intodevice-dependent image signals (C, M, Y) {either one set of imagesignals (Ca, Ma, Ya), (Cb, Mb, Yb), (Cc, Mc, Yc)) as intermediate valuesusing one of the one-dimensional LUTs 16, 18, 20, and thereafterconverting the device-dependent image signals (C, M, Y) as intermediatevalues into block dye density values (C, M, Y) as device-independentimage signals using the polynomial according to the equation (15) set inthe DDC/DIC converter 30, and the output converting process ofconverting the block dye density values (C, M, Y) as device-independentimage signals into device-dependent image signals (C, M, Y, K)corresponding to the output device with the color processor 32.

In the input converting process, device-dependent image signals (R, G,B) are converted into intermediate device-dependent image signals (C, M,Y) as intermediate values using one of the one-dimensional LUTs 16, 18,20, and thereafter the device-dependent image signals (C, M, Y) asintermediate values are converted into block dye density values (C, M,Y) as device-independent image signals using the polynomial (theequation (15)} set in the DDC/DIC converter 30. Therefore, the accuracyof approximation by the polynomial is greatly increased, resulting in alarge increase in the accuracy of conversion by the polynomial.

Heretofore, a large-size input profile has been needed to convert thedevice-dependent image signals (R, G, B) are converted into thedevice-independent image signals (C, M, Y). In the present embodimentwhich uses the polynomial, a small-size input profile may be usedbecause coefficients Bm may be held as a profile. The accuracy ofconversion may be equal to or greater than the conventional accuracy ofconversion by increasing the number of terms of the polynomial.

If the input device remains the same, then it is not necessary to selectone of the one-dimensional LUTs 16, 18, 20, but a single one-dimensionalLUT where the value of the polynomial is most accurate may be fixedlyused.

If the accuracy of conversion does not need to be substantially high orthe approximation by the polynomial is highly acceptable, then theone-dimensional LUTs 16, 18, 20 may be dispensed with, and the outputsignal from the scanner 14 may be supplied directly to the tableselecting calculator 22 and the DDC/DIC converter 30. This modificationhas a simpler arrangement.

In the present embodiment, the color processor 32 performs an imageprocessing process based on the block dye density values (C, M, Y) asdevice-independent image signals. Therefore, the color processor 32 canutilize the image processing technique that uses density-based imagesignals as conventional image processing resources.

The device-independent image signals representing densities with blockdyes have properties similar to conventional density values andequivalent neutral density values. Therefore, the device-independentimage signals can be handled with ease, and can highly accurately beconverted into colorimetric values X, Y, Z through simple calculationsaccording to the equation (5).

In the above embodiment, the device-dependent image signals are C(cyan), M (magenta), and Y (yellow) signals. However, thedevice-dependent image signals may be R (red), G (green), and B (blue)signals, and the device-independent image signals may be C, M, Y signalsrepresenting densities with block dyes, so that R, G, B signals and C,M, Y signals outputted from an image input device such as a digitalcamera, a scanner, or the like may be handled as C, M, Y signalsrepresenting densities with block dyes.

In the above embodiment, a color reversal film is used as a mediumcarrying a color image. However, if a reflective color print is used,then an ANSI/IT8.7/2-1993 chart as an input color target according tothe ANSI may be used.

The above process of converting image signals may be arranged as anapparatus for performing the converting process.

FIGS. 10, 11, and 12 show such image signal converting apparatus 202,206, 202A, respectively, for performing the converting process.

As shown in FIG. 10, the image signal converting apparatus 202 forconverting device-dependent image signals into device-independent imagesignals has an input converter 204 for converting device-dependent imagesignals into device-independent image signals representing densitieswith block dyes.

As shown in FIG. 11, the image signal converting apparatus 206 has aninput converter 204 for converting device-dependent image signalssupplied from an input device which reads an image subject intodevice-independent image signals representing densities with block dyes,and an output converter 208 for converting the device-independent imagesignals into device-dependent image signals for an output device.

As shown in FIG. 12, the image signal converting apparatus 202A has aninput converter 204A for converting device-dependent R, G, B signalsdevice-dependent image signals representing densities with block dyes.

The input device is not limited to the scanner 14, but may be a digitalcamera. The output device is not limited to a printing press, but may bea printer, a CRT display, or the like.

According to the present invention, as described above, device-dependentimage signals are converted into device-independent image signalsrepresenting densities with block dyes.

Since the device-independent image signals representing densities withblock dyes have properties similar to conventional density values andequivalent neutral density values. Therefore, the image processingresources for processing conventional density values and equivalentneutral density values may be employed.

The device-independent image signals can therefore be easily handled byimage processing engineers who have heretofore been involved in imageprocessing tasks using conventional density values and equivalentneutral density values.

Inasmuch as the device-independent image signals representing densitieswith block dyes can highly accurately be converted into colorimetricvalues through simple calculations without the need for integralcalculations, recent image processing resources that processcalorimetric values can also be employed. Accordingly, color managementis facilitated.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

1. A method of converting device-dependent image signals intodevice-independent image signals, comprising the step of: convertingdevice-dependent image signals into device-independent image signalsrepresenting densities with block dyes.
 2. A method of convertingdevice-dependent image signals into device-independent image signals,comprising the steps of: converting device-dependent image signalssupplied from an input device which reads an image subject intodevice-independent image signals representing densities with block dyes;and converting said device-independent image signals intodevice-dependent image signals for an output device.
 3. A methodaccording to claim 1, wherein said device-dependent image signalscomprise R, G, B signals or C, M, Y signals, and said device-independentimage signals comprise C, M, Y signals representing densities with blockdyes.
 4. A method of determining primary colors of a color image,comprising the steps of: setting a color reproduction range of the colorimage on an xy chromaticity diagram; setting three straight linesextending through a chromaticity point corresponding to a standard whiteilluminant on said xy chromaticity diagram and principal wavelengthsrelative to primary colors in said color reproduction range; determiningthe vertexes of a triangle containing said color reproduction range onsaid three straight lines; and determining choromaticity values at thevertexes of said triangle as primary colors.
 5. A method according toclaim 4, wherein said color image is carried on a color reversal film ora reflective color print.
 6. An apparatus for convertingdevice-dependent image signals into device-independent image signals,comprising: an input converter for converting device-dependent imagesignals into device-independent image signals representing densitieswith block dyes.
 7. An apparatus for converting device-dependent imagesignals into device-independent image signals, comprising: an inputconverter for converting device-dependent image signals supplied from aninput device which reads an image subject into device-independent imagesignals representing densities with block dyes; and an output converterfor converting said device-independent image signals intodevice-dependent image signals for an output device.
 8. An apparatusaccording to claim 6, wherein said device-dependent image signalscomprise R, G, B signals or C, M, Y signals, and said device-independentimage signals comprise C, M, Y signals representing densities with blockdyes. 9.-11. (canceled)
 12. The method of claim 1, wherein theconverting comprises converting each color of device-dependent imagesignals into color signals of the device-independent image signalsrepresenting densities with multiple colored block dyes.
 13. Theapparatus of claim 6, said input converter converting each color ofdevice-dependent image signals into color signals of thedevice-independent image signals representing densities with multiplecolored block dyes.
 14. The apparatus of claim 7, converting each colorof device-dependent image signals into color signals of thedevice-independent image signals representing densities with multiplecolored block dyes.